I try to avoid contest. Our district does not endorse contest and I try not
to do them in school either. When special projects come up I either choose
work already done or choose a student who could use the boost. Saves a lot
of hurt feelings. The only time I do a contest like thing is for our art
show poster/invitation design. But all fifth graders participate, it is a
lesson in advertising and logo (similar to what you are doing) and the
students choose one piece to represent their class. We talk about the
objectives, what we are trying to say with the design, etc. I never vote.
This way the kids feel pretty ok about it. Don't know if that helps or not.